Black hole growth at the dawn of the universe: insights from cosmological simulations

Abstract

Observations of supermassive black holes within the first billion years after the Big Bang challenge standard growth models. They appear too massive to have grown at Eddington-limited rates, giving rise to three broad categories of explanation: earlier formation, larger initial seed masses, or periods of super-Eddington accretion. Some recent observations support the latter, but the conditions required to enter this regime and its duration remain unclear. This thesis explores sustained super-Eddington accretion using high-resolution cosmological simulations of small-scale gas dynamics in pre-galactic environments during the Cosmic Dawn. Simulations were performed with the {\tt{Enzo}} code, which I modified to resolve scales relevant for the widely used Bondi-Hoyle-Lyttleton accretion model.

Comparisons between this model and direct inflow measurements under no-feedback conditions show that small black hole seeds ($11 \msun$) struggle to grow, while larger seeds ($270 \msun$) exhibit more stable, convergent growth histories as resolution increases. Black hole thermal feedback was then introduced at varying intensities and resolutions once these black holes had developed a pc-scale disc structure. Super-Eddington growth persisted with certain parameter configurations, establishing that black holes with initial masses of $\gtrsim$$1000 \msun$ can still accrete efficiently over timescales $\lesssim$$1 \Myr$ under effective thermal feedback. The resolution of this subset of super-Eddington simulations was then lowered, and they accreted for $\sim$$10 \Myr$ with thermal feedback activated. Their growth was quickly curtailed, suggesting that either early-stage accretion was resolution-dependent, feedback required longer to become fully disruptive, or the mini-halo failed to accumulate a sufficient gas reservoir for long-term black hole growth. I conclude that black holes with initial masses of $\gtrsim$$1000 \msun$ can achieve super-Eddington accretion, but long-term sustainability is constrained by feedback strength and the host halo’s ability to retain gas. Overall, my results suggest that both larger-than-stellar seed masses and efficient accretion are necessary to form supermassive black holes in the early Universe. High-resolution cosmological simulations that capture the complex interplay between accretion, feedback, and large-scale gas dynamics are critical for extending this work.